Running real rough all of a sudden. Confused.

Hello. First post. I have a 78 R80/7 and everything was going pretty great with the work that I have completed. However, the other day I had to add some brake fluid (recently bled and inadvertently created an air bubble that worked its way out) and after I reinstalled the tank (done many times with no issues) the left fuel petcock started leaking from the fuel line connector threads. Apparently they became stripped as I must have crossed up the threads when re-tightening. I put a temporary fix on with some thread sealer and that fixed the leak. However, when I started the bike back up, it now runs extremely choppy. I had the carbs balanced at the local shop a couple months ago and the bike had been running like a dream up until this. I didn't make any carb adjustments at all and I am wondering if I somehow threw them out of sync because it really seems like it is running on mostly one cylinder. I checked the float bowl levels and everything checks out for a 32mm bing. Any advice?

What often happens is that removing and reinstalling the tank will allow the throttle cables to move a bit and sometimes bind so that they are out of sync and possibly pull one cable tight enough to affect idle. Recheck the cable routing so there is no binding and see what that does.

... it really seems like it is running on mostly one cylinder.

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Well, is it, or isn't it... running on one cylinder? I encountered this same situation and it came down to a clogged main jet. I'd suggest inline fuel filters on each side, if you don't already have them installed.

Check the spark plug on the offending side and see if it is black and wet. Ground the plug and see if it's sparking.

If the cylinder isn't firing you should be able to smell raw fuel at the end of the silencer.

lmo1131 said:

Well, is it, or isn't it... running on one cylinder? I encountered this same situation and it came down to a clogged main jet. I'd suggest inline fuel filters on each side, if you don't already have them installed.

Check the spark plug on the offending side and see if it is black and wet. Ground the plug and see if it's sparking.

If the cylinder isn't firing you should be able to smell raw fuel at the end of the silencer.

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The suggestions so far are good ones, but I suspect that in taking off the tank, you stirred up some dirt in the tank that has been passed along to the carbs. My first suspect is that the carb float shutoff isn't working due to dirt, and allowing one carb to run rich. Drain both fuel bowls, and make sure all dirt is cleaned out and "resurface" each of the float valve seats (I used a little piece of 240 wet-or-dry about the size of a paper-punch hole and put it on the end of a pencil and rotated).

I also recommend getting a couple of in-line filters (best price I found is around $2.00 each from Max's BMW - talk to Rusty). There are some on here who will disagree with this and say you don't need them. DON'T LISTEN TO THEM!! For the cost, this is cheap insurance that you will never get any dirt into the carbs.
I did this (resurface & filters) to my '78 R100/7 and have never had an issue with this again. Carbs have been whistle-clean ever since. I also use LUCAS gasoline additive (I buy in gallons and also use in my car) and it keeps everything clean and lubricated. Mixes at 400:1 so a gallon lasts me over a year!

Why are you unscrewing the petcock? For me, I usually just remove the fuel line..........thread sealer???? what is that? That's a definite jet plugger.

Which cylinder? Easy to just lift the throttle cable a bit and see which one is running

On the offending cylinder take off the float bowl. Fuel level right??.........Blip the float a couple of times and then let fuel flow through the line by letting the float just be open. Put the bowl back on......Run?

"I put a temporary fix on with some thread sealer and that fixed the leak."

Perhaps thread sealer got into the fuel flow into the carb.

If it's running on one cylinder, you may have loosened one of your plug wires at the coil, while removing and refitting the tank. I also agree with the previous poster regarding removing the tank: just pull the fuel hose off of the nipple and leave the petcocks and threaded portions alone. Inline fuel filters are a good addition.

Fixed!

Guys, thanks for all the suggestions. I agree that unscrewing the petcock was the wrong method and I will adjust in the future. It turned out that the throttle cable was the issue. I had checked that after it was suggested and there seemed to be free play at both ends. I moved it around some and had no results. However, in desperation this morning, I really pushed it around and it gave way. It apparently got caught underneath the tank. After freeing it, the bike is back to its old self. Thanks again! You guys saved me a lot of money from a trip to the shop.

Good on you! Thanks for following up. Enjoy you're fall riding.

Petcocks

Just a quick note on petcocks in general. The newer petcocks with the outlet horizontal should have a mesh strainer inside over the two pick-up tubes. This strainer has an aluminum sealing washer at the bottom.
The older models with the vertical spigot have a mesh strainer at the bottom above the spigot. Often this strainer is either missing or stuck up in the petcock body. In any event, the sealing on both top and bottom is NOT done with the threads. On top is a fiber washer that seals the two mating surfaces. On the bottom it depends on the manufacturer. If the spigot is metal then there is a smaller fiber washer sealing the joint. If the spigot is plastic then it in itself does the sealing.
When installing the petcock start by putting the threaded collar on the petcock. All you want to do is just catch the threads without crossing them up. Then install on the tank and evenly tighten the collar, drawing the petcock to the tank. The result will be that you have the maximum amount of thread contact on both the tank and the petcock.

Petcocks

An even easier method of installing the petcocks was passed down to me. Firstly, start the nut on the petcock and turn it in exactly two turns. A light scribe mark or felt pen mark at the nut and petcock will help maintain the orientation. While carefully maintaining that alignment, take the assembly and turn it in to the tank exactly two turns and then snug it up the rest of the way. This way guarantees that the same number of threads is engaged.
Russ

petcocks

1074 said:

Just a quick note on petcocks in general. The newer petcocks with the outlet horizontal should have a mesh strainer inside over the two pick-up tubes. This strainer has an aluminum sealing washe--------------------------------------- without crossing them up. Then install on the tank and evenly tighten the collar, drawing the petcock to the tank. The result will be that you have the maximum amount of thread contact on both the tank and the petcock.

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Good and accurate description, thank you!

Welcome to the forum, Carlrf!!

20774 said:

Welcome to the forum, Carlrf!!

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Thanks, just picked up a couple rough /5s after being w/o a BMW for 10+ years. Both run and licensed but need love.